Radio communication between two terminals is subject to ‘fading’ conditions caused by the constructive addition or cancellation of multiple arriving signals. These signals might be comprised of a direct signal from transmitter to receiver, plus various other signals that arrive at slightly later time (and from different angles), having been reflected from other objects in the path between the two terminals. Dependent on the exact position of the transmitter and receiver terminal, these multiple arrivals will arrive either in-phase (giving constructive addition) or out of phase (giving signal cancellation). This variation in the received signal power is referred to as fading. The extent to which the local environment varies (e.g. due to leaves on trees moving, vehicular movement) determines whether the fade conditions remain constant for a particular placement of the terminals or vary with time.
Typically, a radio link will be deployed with sufficient margin in the received signal strength such that fades due to signal cancellation can be tolerated, while still maintaining sufficient signal power for the transmitted data to be decoded. This allowance has a significant impact on the range that can be achieved with the radio link, for a given transmitted power output level. It is therefore highly desirable to identify techniques which allow this fading margin to be minimised.
One such technique is the use of receive diversity. The receiving terminal is equipped with two antennas which may be positioned, for example, with a spatial separation that is sufficient for the fading conditions at each antenna to be considered statistically independent. In a switched diversity mode of operation, the receiver then selects the antenna with the best signal. If, for example, there is a 1% probability of fades greater than 20 dB below the mean signal power (averaged over local fading), there is then only a 0.01% chance that both antennas will have above a 20 dB fade. For a constant outage probability, the fade margin can therefore be reduced.
FIG. 1 shows a transmitter 101 having two antennas 102, 103 and a receiver 104 having two antennas 105, 106. There are 4 possible propagation paths 108-111 between the transmitter and the receiver antenna pairs. If the transmitter 101 transmits using one of its antennas 102, the receiving terminal (or receiver) can select the better of the two propagation paths 109, 110 to the two receiver antennas, which considerably reduces the fade margin required. This provides a 2-way switched diversity function.
In a time domain duplex (TDD) mode of transmission, the same frequency band is used for the reverse link (terminal B to terminal A) as for the forward link (terminal A to terminal B). For a communication that begins with a link from terminal A to terminal B, it is possible for terminal B to benefit from 2-way diversity. Provided that the propagation conditions have remained constant while the transmission switches direction, terminal B can then re-transmit back to terminal A using the same antenna that was found to be best when it was in receiving mode. Terminal A then makes a second antenna selection of its two antennas for signal reception. When terminal A transmits again back to terminal B, it can again select the best antenna from reception for use as the transmitting antenna. This can continue indefinitely, iterating towards the best possible selection of all four propagation paths, and adapting to changes in the propagation conditions. This process is referred to herein as the “iterative process”.
However, it can be shown that the gain available (i.e. reduction in fade margin) using the iterative process is in many circumstances less than the potential diversity gain if the best of all possible paths were selected.